It is the purpose of this study to explore the plasticity of, developing cerebral cortex during the postnatal period, using the pyramidal motor system of the rat as a model. In the proposed experiments, effects of restriction of motor activity and of sensory .feedback during the postnatal period on number and cortical location of corticospinal (CS) neurons will be studied. The experiments are a direct, systematic extension of work completed during the 01-03 years of the project. During this first phase, it was found that unilateral forelimb restraint between age 5 and 30 days leads to an increase in CS connections and in enlargement of the cortical area of representation of the restrained limb. The proposed studies address several unresolved issues related to effects of the periphery on the anatomy of the pyramidal motor system: 1) What are the age limits of the observed effects? 2) What is the minimum period of limb restraint that produces an effect? 3) Does limb paralysis by neuromuscular blockade produce similar effects? 4) what is the effect of elimination of sensory feedback, both in animals without and with limb restraint? 5) Are increased numbers of CS connections after limb restraint associated with decreased normal cell death in the target zone of CS axons, thereby making more targets available for innervation? 6) Are the same cortical neurons that form an, uncrossed CS tract after neonatal hemispherectomy involved in the formation of extra crossed connections after limb restraint? The basic methodology will be that of identification of CS neurons by unilateral spinal cord injection of a retrograde tracer, followed by computer-- assisted quantitation of number of pyramidal tract cells and construction of maps of cortical representation. The proposed studies are expected to provide important information, concerning the limits and mechanisms of plasticity in the developing motor system. Such knowledge is of major importance for the understanding: of variations in normal development and of developmental errors.